In the prior art, soil-bentonite slurry walls have been employed at hazardous waste sites to restrict the movement of contaminated groundwater. Construction of a typical soil-bentonite slurry wall begins with excavation of a narrow trench which is kept filled with a bentonite slurry to prevent wall collapse. The bentonite slurry deposits a low permeability filter cake at each side of the soil-trench interface. Subsequently, the trench is backfilled with a soil-bentonite mixture to complete the slurry wall.
FIG. 1 depicts a typical prior art slurry wall designated by the reference numeral 10. The slurry wall includes opposing low permeability filter cakes 1, generally less than 3 mm thick, and a soil-bentonite backfill layer 3 disposed therebetween, generally 1 meter thick. The filter cakes 1 play an important role in the formation of permeability of a completed slurry wall. The presence of the filter cakes ensures that the overall permeability remains below 10.sup.-6 cm/s even when using a soil-bentonite backfill having a permeability of only 10.sup.-4 cm/s.
The slurry walls 10 with earthen caps 13 are formed to extend from the surface 5 to the native impervious layer 7 and surround the waste site 9. Optionally, an extraction well 11 is installed between the slurry wall 10 and the waste site 9 to create a groundwater flow toward the waste site. This flow of groundwater towards the waste site, (negative advection as indicated by the arrow so labeled in FIG. 1) diminishes advective transport or diffusion (indicated by the opposing arrow labeled diffusion) of contaminants from groundwater above or below the water table 15 through the soil-bentonite slurry wall 10 to the surroundings.
However, the slurry walls shown in FIG. 1 are not totally effective in controlling the diffusive migration of organic or inorganic solutes through the slurry wall. Although the advective transport or diffusion of the contaminated groundwater moving away from the waste site can be slowed substantially when hydraulic gradients using extraction wells or the like are created toward the contaminated subsurface regions, the diffusive transport of dissolved waste through these low permeability barriers can still be significant over time. Particularly troublesome are low molecular weight organic solutes.
Low molecular weight organic solutes can have effective molecular diffusion coefficients only two to four times less in soil-bentonite mixtures than in a free aqueous solution. This small difference between a free aqueous solution and the low permeability structure indicates that slurry walls cannot effectively control the diffusive migration of these types of waste materials.
In response to these deficiencies, various adsorptive materials such as fly ash, zeolite and organically modified clay have been proposed for attenuating contaminant movement through waste containment systems. U.S. Pat. No. 5,132,021 to Alexander discloses in-situ treatment of waste water to prevent groundwater contamination which suggests the addition of a contaminant neutralizing or removing material mixed with a water-swellable clay. These adsorbent materials include a wide variety of materials such as natural or synthetic zeolites, organophilic clays, silica hydrogel-base compositions, fibers such as micro-crystalline cellulose and carbon or activated carbons, for example, activated charcoal.
These prior art methods have not been totally successful in solving the problem of organic waste transport through these types of clay walls. For example, the use of fly ash can create other adverse impacts on the environment due to the presence of toxic trace metals, especially complexes of arsenic, selenium and chromium.
The organically-modified clays may also be ineffective in controlling certain specific organic compounds. Depending on what type of organic cation is used, different organically modified clays modify different adsorption preferences for specific organic compounds. Thus, one type of organically-modified clay may not be particularly adapted for adsorption of a particular organic or inorganic contaminant.
As such, a need has developed to provide an improved waste containment wall construction and composition which is effective against a wide range of organic and inorganic compounds, in particular, low molecular weight organic contaminants such as trichlorethylene and dichlorobenzene.
In response to this need, the present invention provides an improved low permeability waste containment construction and composition using granular activated carbon. The proper use of granulated activated carbon in clay-containing waste containment structures does not affect the characteristics of structures such as slurry walls including permeability and thus provides unexpected effectiveness in controlling transport of waste organic and inorganic contaminants through the clay-containing wall structures.